The invention relates to an apparatus for developing an electrostatic latent image, and more particularly, to such apparatus which forms a toner image by applying a dry magnetic two-component developer including a magnetic carrier and a toner to an electrostatic latent image formed on a charge retaining member.
FIG. 1 shows an example of a conventional apparatus for developing an electrostatic latent image which forms a toner image by utilizing a magnetic two-component developer. In this example, a photosensitive member for electrophotography is formed by a charge retaining member including an electrostatic recording medium which is applied to the surface of a drum-shaped, conductive support (not shown). An electrostatic latent image is formed on the surface of the retaining member 1 by a technique which is known in itself. A magnet roller 2 is disposed within a cylindrical, non-magnetic sleeve 3, which is closely spaced from the charge retaining member 1, being a loose fit therein. A scraper 4 has its one end disposed in contact with the non-magnetic sleeve 3 to remove developer from the sleeve and to guide it away from the latter. An agitator blade assembly 5 is rotatably mounted at a point below the other end of the scraper 4. The sleeve 3, scraper 4 and agitator blade assembly 5 are housed with a developer housing 8, which additionally includes a doctor blade 6 disposed in a region along the lower edge which is lccated below the spacing between the sleeve 3 and the charge retaining member 1. The housing 8 is also formed with an opening in its top wall, located above the agitator blade assembly 5, for receiving a toner replenishing roller 7 in a rotatable manner. A toner hopper 9 is disposed on top of the housing 8 and receives the roller 7 in the bottom thereof.
A developing apparatus of the prior art constructed as mentioned above has exhibited a good match with existing developers, and a variety of arrangements are utilized in implementation. However, a plurality of drawbacks are found with such arrangement. Existing magnetic, two-component developer is of a reduced durability and hence has a lifetime which is less than desired, thereby requiring frequent replacement of developer. In addition, the performance of toner and carrier is less than satisfactory, and hence an adequate agitation and mixing to maintain a proper mixing ratio is necessary to assure a favorable developing effect. Furthermore, the toner tends to be scattered. To prevent the described drawbacks from occurring, a conventional developing unit which utilizes a magnetic two-component developer requires, in addition to those members such as magnet roller or non-magnetic sleeve which are directly used to effect a developing process, a developer receiving chamber of an increased size in order to avoid frequent replacement of developer or components which are used to remove developer from the sleeve 3 and to mix and agitate it uniformly with a toner which is replenished anew, thus resulting in a complex and large sized arrangement.
However, recent improvement in the technology relating to the quality of toner and carrier has greatly improved the durability of developer while simultaneously increasing the tolerance with respect to a variation in the mixing ratio of toner. It will be noted that a conventional developing apparatus is not designed for use with such improved developer, which caused problems.
Improvements relating to developer include a resin coating of carrier and the implementation of the ferrite carrier, as major improvements. The resin coating stabilizes the characteristics of a developer against variations in the environment of use thereof and also stabilizes the toner charging effect while providing a remarkably improved durability of developer. The use of the ferrite carrier achieves similar effects while it operates on the basis of a different principle. Such improvement relating to developer has increased its lifetime to a level which can be exemplified by the production of 50,000 to 100,000 copies per 500 g to 1 kg of developer. It is found that such lifetime may be excessively long for copying machines which produce a reduced number of copies monthly.
While it is contemplated that a proper lifetime may be attained for the developer by reducing the amount of developer which is loaded, it is to be noted that the design of a conventional developing apparatus is such that a developer is once removed from a sleeve for mixing and agitation, so that it is impossible to reduce the amount of developer loaded below a value which is required to enable a normal operation of such mixing and agitating mechanism. It may also be contemplated to provide a developing apparatus having reduced dimensions for various parts, but the configuration of a magnet roller cannot be simply reduced in size because of its relation to the developing performance.
As another example, FIG. 2 shows an apparatus for developing an electrostatic latent image which includes a toner mixing ratio detector 10 and a controller 11 which controls the replenishment of toner for maintaining a constant mixing ratio of toner in a magnetic, two-component developer. In this apparatus, only the toner will be consumed as a developing process proceeds, and accordingly, a toner mixing ratio in the developer is detected and a resulting signal controls the rotation of a toner replenishing roller 7 or the amount of toner replenished. The detector 10 comprises a piezoelectric vibrator connected to a diaphragm which is integral with a weir plate partly intercepting a flow of developer. The detector is mounted within the developer housing 8 at a location above the non-magnetic sleeve 3. The detector 10 produces an electrical signal indicating a change in the amplitude of the diaphragm which occurs as the amount of developer which remains stagnant on the weir plate varies in accordance with a change in the fluidity or the bulk specific gravity of developer resulting from a change in the toner mixing ratio. The signal from the detector 10 is fed through the controller 11 to cause the toner replenishing roller 7 to rotate, by actuating a motor, a clutch or the like.
As described, in conventional arrangements, it is necessary to detect a toner mixing ratio in the developer or to detect the optical density of a developed image in order to produce a control signal which is used to control the replenishment of toner. This complicates the arrangement of the developing apparatus, standing in the way of providing a developing apparatus of simple arrangement.
Another difficulty results from the usual construction of the toner replenishing roller 7. Generally, the roller 7 is provided with a groove or grooves, is knurled or is formed of sponge material so as to exhibit unevenness in its surface so that the toner is introduced into such recesses when the latter faces the toner hopper 9, and is released or caused to fall into the housing 8 by gravity or by means of a scraper (not shown) whenever such recesses have rotated to face the interior of the housing 8. Accordingly, the toner is in the form of small agglomerates when supplied, and cannot be easily dispersed into the developer in a uniform manner. This causes toner to be produced which is poorly charged, giving rise to the occurrence of a fogging over a non-image area or a scattering of toner. Furthermore, a conventional developing apparatus requires the provision of an agitator assembly of an increased size to achieve a uniform dispersion of toner replenished into the developer, and also requires that an increased amount of developer be used to produce a constant flow of developer within the apparatus.